1. Field of the Invention
This invention relates to low free formaldehyde and low N-methylol group containing melamine derived crosslinker compositions and to low formaldehyde emitting thermosetting coating compositions based on these crosslinkers compositions, which coating compositions can provide durable, substantially defect-free films upon curing. The low formaldehyde emitting thermosetting coating compositions in accordance with the present invention contain, in their broadest concept, a low free formaldehyde and low N-methylol group containing partially alkoxymethylated high NH (high imino) melamine crosslinking agent and a polyfunctional material containing functional groups capable of reacting therewith.
2. Description of Related Art
Currently, the majority of important industrial coatings include melamine-formaldehyde crosslinked liquid systems. Such liquid coatings which employ partially or fully alkoxymethylated amino resin crosslinkers are in general well-known to those of ordinary skill in the art, as exemplified by numerous references including, for example, U.S. Pat. Nos. 4,081,426, 4,101,520, 4,129,681, 4,243,705, 4,276,212, 4,330,458, 4,374,164, 4,433,143, 4,425,466, 4,873,298, 5,155,201, JP-A-51125102 and GB-A-2254328, all of which are incorporated herein by reference for all purposes as if fully set forth. These and other references disclose a wide variety of backbone resins, catalysts, additives and amino resin crosslinkers therefor including, for example, tris- and tetra-substituted melamines for coatings applications. The present invention is concerned with liquid amino resin crosslinkers and, in particular, with the alkoxymethyl substituted melamine derived crosslinkers.
A problem with melamine formaldehyde resins is that they emit formaldehyde during the curing process, creating potential health and safety hazards and environmental pollution problems. Because of the health, safety and environmental concerns associated with large scale use of compositions containing melamine-formaldehyde crosslinking agents, demand for lower formaldehyde emitting coatings has steadily increased in recent years providing an incentive to find a practical solution to the problem.
One common approach to solving this problem has been the use of low free formaldehyde containing melamine derived crosslinking agents. While employing low free formaldehyde crosslinkers has been generally helpful to some extent, the improvement has been modest because preparation of low free formaldehyde containing crosslinkers has been difficult and emission problems have persisted even when low free formaldehyde containing crosslinkers are employed. Surprisingly, formaldehyde emissions during cure are greater than the actual free formaldehyde levels present in the starting coatings compositions. The origin of the excess formaldehyde released during cure has not been completely elucidated to date and, as a result, the problem of formaldehyde discharge into the environment continues to concern the coatings industry.
It has now been surprisingly discovered, in accordance with the present invention, that formaldehyde emission levels of well-known melamine-formaldehyde crosslinked systems can be reduced by replacing the previous melamine formaldehyde crosslinkers with a certain class of low free formaldehyde, low N-methylol and relatively high NH group containing liquid alkoxymethylated melamine resins. Curable compositions containing these crosslinkers of the present invention cure rapidly under normal cure conditions to produce durable films without emitting unacceptably high levels of formaldehyde.